This invention relates to a process for the production of alcohol (ethanol) by the continuous fermentation of a must being circulated in a closed circuit by a mammoth (air lift) pump, with fresh broth being continuously injected into the circuit and fermented broth being continuously removed from the circuit.
This invention also relates to a fermenter for the continuous production of alcohol, comprising a fermentation vat and a mammoth pump which are connected in a closed circuit, a device for injecting fresh broth and a device for removing fermented broth.
Several processes and fermenters for the continuous production of alcohol are known, in which the circulation of a culture medium rich in fermentable sugar and which is designated in the present specification by the term "must" is assured either by means of an ordinary pump, or by a mammoth pump fed via a compressor. A process is known in which the circulation is assured solely by the release of carbon dioxide gas produced during fermentation.
In this last-mentioned known process, the fermentation conditions vary considerably the further one moves up the fermentation vat, because of the expansion and the coalescence of ascending bubbles of carbon dioxide gas and, even if the yield in weight of alcohol obtained with respect to the weight of fermentable sugar used is good, the productivity is low compared to the size of the installation.
In the other processes mentioned above, it is possible to obtain suitable productivities as well as good yields which approach the theoretical maximum. The dilution rates, that is the ratio between the quantity of fresh must which is injected per hour and the quantity of must contained in the circuit, are fairly high, of the order of ten percent or multiples thereof. The active yeast content of the must is about 50 g of dry matter of yeast per liter, the fresh must which is injected contains from about 100 to 150 g of fermentable sugars per liter and the fermented must which is removed contains from about 6.5 to 8.5% of alcohol. All these quantities are substantial and are to be found in the best of these known processes and fermenters.
However, the dimensions of these fermenters and the quantities of must which are treated by these known processes are relatively modest, i.e., from a few liters to a few m.sup.3. The reason for this lies in the fact that it is very difficult to maintain homogeneous conditions favourable for alcoholic fermentation in a fermenter of several tens, even hundreds, of m.sup.3 without running into serious technical and economic difficulties. In effect, if good transfer to the yeast solely of the oxygen necessary for its anaerobic metabolism is to be supplied, it is necessary to assure on the one hand good agitation of the medium and, on the other hand, an adequate oxygen concentration in the gaseous phase. By increasing the dimensions of the fermenter, the inhomogeneity of the conditions prevailing in the fermentation medium are increased, notably as a function of the height of the vat. The restoration of this homogeneity would imply for the known fermenters and processes complicated devices for the injection of gas under a variable pressure and/or differential stirring devices distributed regularly over the complete height of the vat. The maintenance of this homogeneity could only be ensured by the expenditure of a very great stirring energy, at a prohibitive cost.